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HTML

<input type="text" id="ga" value="UA-1234-12" placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="YT-1234-12" placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="UA-1234-1" placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="MO-1-1" placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="ua-1-1" placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="UA-1234-"  placeholder="Enter Google Analytics ID" />
<input type="text" id="ga" value="UA--1" placeholder="Enter Google Analytics ID" />

JavaScript

function validateGA(el) {
    var regexp = /(UA|YT|MO)-\d+-\d+/i;
    return regexp.test(el.value);
}

function colorField(el, res) {
    el.style["borderColor"] = res ? "green": "red";
}
var ga = document.getElementsByTagName('input');

for (var i = 0; i < ga.length; i++) {
  colorField(ga[i], validateGA(ga[i]));
}